A superabsorbent polymer (SAP) is a synthetic polymeric material capable of absorbing moisture at about 500 to 1000 times its own weight, and is also called a SAM (super absorbency material), an AGM (absorbent gel material), etc. Since superabsorbent polymers started to be practically applied in sanitary products, they have now been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for civil engineering and construction, sheets for raising seedlings, fresh-keeping agents for food distribution fields, and the like.
As a preparation process for such superabsorbent polymers, a process of reverse phase suspension polymerization and a process of solution polymerization have been known.
Of them, preparation of the superabsorbent polymer by reverse phase suspension polymerization is disclosed in, for example, Japanese Patent Laid-open Publication Nos. S56-161408, S57-158209, S57-198714, etc.
Further, preparation of the superabsorbent polymer by the solution polymerization further includes a thermal polymerization method in which a water-containing gel polymer is polymerized while being broken and cooled in a kneader equipped with a plurality of shafts, and a photo-polymerization method in which an aqueous solution with a high concentration is irradiated with UV rays on a belt to be polymerized and dried at the same time.
Meanwhile, in the preparation process of the superabsorbent polymer, fine particles are inevitably formed during a process of pulverizing the polymer. These fine particles function to deteriorate physical properties of the superabsorbent polymer, and the fine particles are separately recovered through size-sorting after pulverization and then reused. For example, a method of adding the fine particles during formation of a water-containing gel polymer has been suggested. However, when the fine particles are added during formation of the water-containing gel polymer, the fine particles scatter light to interfere with polymerization, and uneven polymerization is problematically induced to deteriorate physical properties of the resin.
Therefore, a method of regranulating the fine particles in a separate process has been suggested. This regranulating is a method of forming granules having a larger particle size than the fine particles by mixing the fine particle with a solvent. However, the fine particle granules formed by this method do not show a sufficient binding force between fine particles, and thus there is a disadvantage that most granules are easily re-broken. Accordingly, when the fine particle granules are used in the preparation of the superabsorbent polymer, there is a problem that the fine particle granules are re-broken to deteriorate physical properties of the polymer.